[ovs-discuss] [OVN] Packets flooded when using VLAN backed networks

Daniel Alvarez Sanchez dalvarez at redhat.com
Fri Sep 11 15:57:45 UTC 2020

Hi folks,

This is probably not a bug and not sure if much can be done about it but
thought of raising it here for discussion.

I have deployed a simple topology with two logical switches (VLAN backed
network), a logical router and a couple of VMs. When pinging between the
logical switches, all the traffic is flooded in the upstream switch.

Example using this logical [0] and physical [1] topologies, when pinging
from vm3 (worker2) to vm1 (worker1) and capturing traffic on host1:

15:50:18.790323 1e:02:ad:bb:aa:dd > 40:44:00:00:00:01, ethertype 802.1Q
(0x8100), length 102: vlan 190, p 0, ethertype IPv4, (tos 0x0, ttl 63, id
47366, offset 0, flags [DF], proto ICMP (1), length 84) > ICMP echo request, id 1671, seq 11, length
15:50:18.790428 1e:02:ad:bb:aa:77 > 40:44:33:00:00:03, ethertype 802.1Q
(0x8100), length 102: vlan 170, p 0, ethertype IPv4, (tos 0x0, ttl 63, id
44948, offset 0, flags [none], proto ICMP (1), length 84) > ICMP echo reply, id 1671, seq 11, length 64

The reason is that, as we translate the eth.src to that of the
"ovn-chassis-mac-mappings", the ToR will never see a packet whose eth.src
is either vm1 or vm3 so it'll never learn their addresses and flood the
traffic to all ports.

In the example above:

[root at worker1 ~]# ovs-vsctl get open . external_ids:ovn-chassis-mac-mappings

[root at worker2 vagrant]# ovs-vsctl get open .

I understand that the benefit of using the ovn-chassis-mac-mappings is the
distributed routing capabilities but I wonder if we could come up with a
way of avoiding the flood.

In case somebody's interested in replicating this scenario, you can find a
vagrant setup here [2].


[0] http://dani.foroselectronica.es/wp-content/uploads/2020/09/extp_log.png
[1] http://dani.foroselectronica.es/wp-content/uploads/2020/09/expt_phy.png
[2] https://github.com/danalsan/vagrants/tree/master/ovn-external-ports
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